This application claims priority under 35 U.S.C. Sec. 119 to No. 11-174198 filed in Japan on Jun. 21, 1999, the entire content of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to an apparatus for determining a state of braking operation of a vehicle, and more particularly to the apparatus for determining whether a rapid operation of a brake pedal was performed or not, which is applicable to a front-rear braking force distribution control system for controlling the braking force applied to a rear wheel to be in a certain relationship with the braking force applied to a front wheel in accordance with a result determined by the apparatus for determining the state of braking operation.
2. Description of the Related Arts
In general, when a moving vehicle is braked, axle loads applied to the front and rear portions of the vehicle respectively will be different from each other due to the moving load caused by the braking operation. Therefore, the braking force applied to front wheels and the braking force applied to rear wheels for locking all of the wheels simultaneously are not in direct proportion to each other, but in such a relationship as called an ideal braking force distribution, which may be varied depending upon the presence or absence of the load on the vehicle. If the braking force applied to the rear wheels exceeds the braking force applied to the front wheels, the directional stability of the vehicle will be deteriorated. In order to keep the braking force applied to the rear wheel lower than that applied to the front wheel and provide a distribution in close proximity to the ideal braking force distribution, a proportioning valve is provided between the rear wheel brake cylinders and a master cylinder. According to this arrangement, the distribution characteristic has a pair of lines having different gradients from each other. When the difference of the loads applied to the inner and outer wheels of a turning vehicle is taken into consideration for example, it is necessary to reduce the braking force applied to the rear wheels much lower than the braking force applied to the front wheels. Therefore, in the case where it is so arranged that the hydraulic braking pressure is always restricted by the proportioning valve, the braking force distributed to the rear wheels will be reduced, so that a large depressing force to a brake pedal will be needed to ensure a desired deceleration, and an excessive load will be applied to the front wheels.
In order to solve the problem as described above, the U.S. Pat. No. 5,624,164 discloses a braking force distribution control system, which is adapted to control the braking force applied to the rear wheel in a predetermined relationship with the braking force applied to the front wheel, by controlling pressure control valves disposed between the wheel brake cylinder and a hydraulic booster or the master cylinder, and controlling a changeover valve disposed upstream of the control valves. Furthermore, in Japanese patent Laid-open Publication No.8-108841, there is proposed a braking force distribution control system which is capable of performing the braking force distribution properly in accordance with a turning state of a vehicle.
With respect to the braking force distribution control system such as those described above, a rear wheel braking force control apparatus is proposed in Japanese Patent No.2591389, in order to improve braking stability by making a braking force distributed to a rear wheel when the braking force applied to the rear wheel is sufficient, larger than that when the braking force applied to the rear wheel is not sufficient, in response to barking conditions such as a road condition or vehicle running condition. The apparatus is provided with a proportioning valve which is disposed in a passage for transmitting a master cylinder pressure to a rear wheel brake cylinder, to control the wheel cylinder pressure in such a manner that variation of the wheel cylinder pressure in response to the variation of the master cylinder pressure will be made small in a region where the master cylinder pressure is more than a first predetermined pressure, and provided with a switching valve which is disposed in a passage for bypassing the proportioning valve and transmitting the master cylinder pressure to the rear wheel brake cylinder. The switching valve is closed when the master cylinder pressure detected by a master cylinder pressure sensor is larger than a second predetermined pressure which is larger than the first predetermined pressure, while the switching valve is opened when the master cylinder pressure is smaller than the second predetermined pressure. And, when the locking limit for the rear wheel is low, the switching valve is closed irrespective of the master cylinder pressure detected by the master cylinder pressure sensor.
Japanese patent Laid-open Publication No.10-138895 discloses a vehicle brake apparatus, wherein a wheel speed difference between a larger one of front wheel speeds and each rear wheel speed is detected, when a vehicle deceleration and a vehicle speed exceed each reference value respectively. When the difference is greater than a first predetermined value, a solenoid valve device is switched to hold a hydraulic brake pressure, and when the difference is smaller than a second predetermined value, the solenoid valve device is switched to increase the hydraulic brake pressure again, thereby to control the braking force distribution of the front wheel and that of the rear wheel independently to approximate to an ideal curve.
According to the front-rear braking force distribution control apparatuses as described above, when the brake pedal is depressed gradually, the braking force applied to the front and rear wheels is distributed properly in accordance with a desired ratio. However, when the brake pedal is depressed more rapidly than a load is shifted in a normal braking operation, so that the rapid operation of the brake pedal is performed, then a control for distributing the braking force applied to the front and rear wheels will be delayed. This is because when the brake pedal is depressed rapidly, the front wheel cylinder pressure will reach a pressure for enabling the wheel to be locked before a desired load shift will occur, to result in a large drop of the front wheel speed, so that the condition for beginning the braking force distribution control will not be fulfilled, although it is generally constituted in such a manner that the braking force distribution control will begin when the rear wheel speed becomes to be smaller than the front wheel speed by more than a predetermined value. Thereafter, the load will be shifted to the front wheel to reduce the rear wheel speed, and when the rear wheel speed will be smaller than the front wheel speed by more than the predetermined value, the braking force distribution control will finally begin.
FIG. 5 shows variations of front and rear wheel speeds, and the difference between the wheel speeds obtained when the brake pedal is rapidly depressed. When the brake pedal is rapidly depressed at the time xe2x80x9ct0xe2x80x9d, a reduction of the front wheel speed Vwfs becomes to be greater than the reduction of the rear wheel speed Vwr*, so that the braking force distribution control will begin at the time xe2x80x9ct2xe2x80x9d when the rear wheel speed Vwr* will become to be smaller than the front wheel speed Vwfs by more than a predetermined value K2 (positive value), thereby to prolong the period after the brake pedal is depressed until the braking force distribution control will begin. Referring to FIG. 6, when the brake pedal is rapidly depressed, a front wheel cylinder pressure will vary as shown by a solid line (Pfe) in FIG. 6, whereas the front wheel cylinder pressure will vary as shown by a broken line (Pfs) when the brake pedal is depressed normally. That is, when the brake pedal is depressed normally and the braking force is applied to the front and rear wheels, the front wheel cylinder pressure Pfs will reach a locking pressure Pkb, in such a state that a load has been shifted from the rear wheel to the front wheel sufficiently to cause a load xe2x80x9cLbxe2x80x9d be shifted to the front wheel. However, when the brake pedal is depressed rapidly, the front wheel cylinder pressure Pfe will reach a locking pressure Pka, in such a state that the load has not been shifted from the rear wheel to the front wheel sufficiently to cause only a load xe2x80x9cLaxe2x80x9d be shifted to the front wheel. Therefore, the front wheel speed Vwfs will become to be smaller than the rear wheel speed Vwr* as shown in FIG. 5, the condition for beginning the braking force distribution control will not be fulfilled for a relatively long time. In FIG. 6, the locking pressure Pke indicates a value obtained when no load shift occurs.
In the Japanese Patent No.2591389, however, nothing is disclosed about the rapid operation of the brake pedal as described above. And, the apparatus as disclosed in the Japanese Patent No.2591389 is to be provided with a pressure sensor in addition to wheel speed sensors, thereby to cause a cost-up. On the contrary, in the Japanese patent Laid-open Publication No.10-138895, it is disclosed that the vehicle deceleration, the vehicle speed, and the wheel speed difference between the front and rear wheels are employed. In this publication, however, nothing is disclosed about the rapid operation of the brake pedal, so that the aforementioned problem can not be solved.
Accordingly, it is an object of the present invention to provide an apparatus for determining a state of braking operation of a vehicle, wherein it can be determined whether a rapid operation of a brake pedal was performed or not, without any particular sensors.
It is another object of the present invention to provide a front-rear braking force distribution control system, wherein it can be determined whether the rapid operation of the brake pedal was performed or not, without any particular sensors, and based upon the determined result, a braking force applied to a rear wheel can be adjusted properly to be in a certain relationship with the braking force applied to a front wheel.
In accomplish the above and other objects, an apparatus for determining a state of braking operation of a vehicle includes a wheel speed detection device for detecting each wheel speed of a front wheel and a rear wheel of the vehicle, and includes a wheel speed difference calculation device for calculating a wheel speed difference between the wheel speed of the front wheel and the wheel speed of the rear wheel detected by the wheel speed detection device. The apparatus further includes a braking operation detection device for detecting an operation of a brake pedal of the vehicle, and includes a rapid operation determination device, which determines whether the brake pedal was rapidly depressed on the basis of the difference calculated by the wheel speed difference calculation device and the operation of the brake pedal detected by the braking operation detection device.
In the apparatus, it is preferable that the rapid operation determination device determines that the brake pedal was rapidly depressed, in the case where it was determined that the wheel speed of the front wheel exceeded the wheel speed of the rear wheel after it was determined on the basis of the difference calculated by the wheel speed difference calculation device that the wheel speed of the front wheel was lower than the wheel speed of the rear wheel by more than a predetermined difference, within a predetermined time period after the braking operation detection device detected the operation of the brake pedal.
The apparatus may further comprise a vehicle deceleration calculation device for calculating a vehicle deceleration on the basis of the wheel speeds detected by the wheel speed detection device. In this case, the rapid operation determination device is adapted to determine that the brake pedal was rapidly depressed, in the case where it was determined on the basis of the deceleration calculated by the vehicle deceleration calculation device that the vehicle deceleration was greater than a predetermined deceleration, and it was determined that the wheel speed of the front wheel exceeded the wheel speed of the rear wheel after it was determined on the basis of the difference calculated by the wheel speed difference calculation device that the wheel speed of the front wheel was lower than the wheel speed of the rear wheel by more than a predetermined difference, within a predetermined time period after the braking operation detection device detected the operation of the brake pedal.
The above-described apparatus for determining a state of braking operation of the vehicle may be incorporated into a front-rear braking force distribution control system, which includes wheel brake cylinders operatively mounted on the front and rear wheels for applying the braking force thereto, respectively, a pressure generating device for pressurizing brake fluid in response to operation of the brake pedal to generate a hydraulic braking pressure and supply the hydraulic braking pressure to each of the wheel brake cylinders, and a pressure control device disposed between the pressure generating device and the wheel brake cylinders for controlling the hydraulic braking pressure in each of the wheel brake cylinders. And, a braking force control device is provided for controlling the pressure control device at least on the basis of the result determined by the rapid operation determination device, and in response to the wheel speeds detected by the wheel speed detection device, and controlling the hydraulic braking pressure in each of the wheel brake cylinders to adjust the braking force applied to the rear wheel in accordance with a predetermined relationship with the braking force applied to the front wheel.